The proposed investigations are directed towards in-depth study in animal and human intestine of the oppositely directed absorptive and secretory processes which together determine net intestinal transport of water and electrolytes. These studies will evaluate: 1) the place of the intestinal mucosal enzyme Na-K-ATPase in basal absorptive and secretory processes in vivo, and in active Na transport in vitro; 2) New aspects of the pathophysiology of diarrhea. Specifically, the mechanism of serotonin-induced water and electrolyte secretion in the rabbit ileum; particularly whether active electrolyte secretion, altered intestinal permeability, or altered neural control are responsible. The methods that will be used to study water and electrolyte transport and the enzymatic mediators involved will include: a) in vivo, the single pass perfusion technique; b) in vitro, the Ussing chamber and voltage clamp technique; and c) determination of activities of the intestinal mucosal enzymes: Na-K-ATPase, Mg-ATPase, adenylate cyclase guanylate cyclase and cyclic nucleotide phosphodiesterase.